Erector spinae plane block (ESPB) is a novel interfascial block technique first described by Forero in 2016. It has been reported to be effective in various clinical indications. In ESPB, local anesthetic is injected between the deep fascia of the erector spinae muscle and the transverse process of the vertebra, allowing cephalocaudal spread along the erector spinae muscle. The goal of the injection is to provide sensory blockade by affecting the anterior rami of the spinal nerves in the region.

ESPB has been used during breast and axillary lymph node surgeries to manage postoperative pain. In addition to tumor cells, immune and inflammatory cells such as neutrophils, platelets, and lymphocytes contribute to the dissemination and invasion of tumor cells into the peripheral circulation. Noninvasive biomarkers such as serum inflammatory markers are increasingly valued for their simplicity and predictive potential. Platelets have been shown to facilitate epithelial-mesenchymal transition of tumor cells; neutrophils support tumor adhesion via growth factors such as VEGF and proteases; lymphocytes play a role in modulating the host immune response to malignancy.

The systemic immune-inflammatory index (SII), calculated as (platelet count × neutrophil count) / lymphocyte count, is a relatively new composite marker that reflects immune and inflammatory responses. It has been evaluated as a prognostic biomarker in several cancer-related studies.

Postoperative pain and surgical stress activate systemic inflammatory responses. ESPB is performed as part of postoperative analgesia in patients undergoing breast cancer surgery under general anesthesia, based on voluntary participation. The aim of this study is to evaluate the effect of ESPB on postoperative SII levels in comparison to other analgesia modalities.

Inclusion Criteria:

• Female patients aged 18-75 years
• ASA (American Society of Anesthesiologists) physical status I-II-III
• Undergoing unilateral breast cancer surgery under general anesthesia
• Volunteered to participate in the study and signed informed consent

Exclusion Criteria:

• Allergy or contraindication to local anesthetics
• Coagulopathy or on anticoagulant therapy
• Infection at the injection site
• History of chronic pain or opioid use
• Neurological or psychiatric disorders affecting pain perception
• Refusal to receive regional anesthesia

Female (because it is a breast cancer study)

Due to privacy and confidentiality concerns, individual participant data (IPD) will not be made available to other researchers. The data will remain confidential, in accordance with institutional and regulatory guidelines.